Enter the Quark

Despite Gell-Mann's pattern
and the classification of particles as either Baryons or Mesons, physicists
were still stuck with hundreds of "fundamental" particles.

To escape the "Particle Zoo," the next logical step was to investigate
whether these patterns could be explained by postulating that all
Baryons and Mesons are made of other particles. These particles were
named Quarks.

Murray Gell-Mann and George Zwieg proposed that baryons are particles
consisting of three quarks (qqq), while mesons are particles consisting
of a quark and anti-quark (q q-bar).

(An "up" quark is labeled by the letter u and the "down" quark by
d. The words up and down are just names for these quarks, nothing
is actually up or down.)